A photocurable ink is fast in drying and includes no volatile solvent, is free from the volatilization of a component hazardous to the environment, and has an excellent performance such as the capability of printing on various substrates; thus, photocurable inks are utilized, for example, in wide fields of various coatings and inkjet printing, in addition to the fields of offset printing, gravure printing, screen printing and letterpress printing. In particular, inkjet printing is capable of forming images simply and inexpensively without distinction of the materials or the shapes of the substrates, and hence is applied to various fields of usual printing of, for example, logotypes, graphics and photographic images, and also specific printing of, for example, marking and color filter. Inkjet printing is expected to yield more satisfactory printed matter, as a result of the effect due to combination with the performance of photocurable ink.
Recently, photocurable ink has come to be required to be also capable of printing, by an inkjet printing method, on substrates to be subsequently subjected to stretching or bending processing.
However, for such requirements, cured coatings of photocurable inks using conventional monofunctional monomers and multifunctional monomers frequently provide too high coating hardness. Accordingly, when stretching or bending processing is performed after the printing, cured coatings cannot follow the stretching or deformation of the substrates, and the adhesiveness of the cured coatings tend to be degraded, to lead to a problem such that the stretching and bending processability of the printed matter is lowered.
Accordingly, in order to solve these problems, for example, Patent Literature 1 proposes an active energy line-curable ink using a monofunctional monomer and a multifunctional monomer in combination, and including as a monofunctional monomer a monomer selected from a phenoxy group-containing (meth)acrylate, an ethylene oxide adduct of a phenoxy group-containing (meth)acrylate and a propylene oxide adduct of a phenoxy group-containing (meth)acrylate. Certainly, the active energy line-curable ink described in Patent Literature 1 is regarded to have a satisfactory stretchability, namely, a satisfactory ductility, but is clearly not an ink pursuing excellence in punching processability.
In this regard, the invention described in Patent Literature 2 is also the same as the invention in Patent Literature 1; although the invention of Patent Literature 2 is an invention to improve the high-temperature stretchability or the like of a photocurable inkjet printing ink composition on the premise of inclusion of urethane oligomer and isobornyl acrylate, nothing is pursued with respect to, for example, the punching processability of the cured ink composition.
However, recently a market demand for, for example, the capability of punching processing with punch or the like has grown. With respect to such a demand, in the case where a plastic sheet to be a printing substrate is thick and is hardly deformed and the shape of the hole to be drilled is as simple as a circle, a cutting processing such as drilling can also adopted, and hence such a processing is possible when the ink is sufficiently allowed to adhere to the printing substrate.
However, there occurs a problem that cutting processing takes very much labor and time, and is not adaptable to objects made of easily deformable materials. In contrast to this, punching processing is characterized by being capable of simply drilling a hole complicated in shape, and hence punching processing comes to be adopted as long as punching processing is applicable.
In this processing, the shear stress is instantaneously exerted on the portion to be punching-processed of the plastic sheet (printing substrate). Accordingly, the ink coating printed in the vicinity of the punching-processed portion is required to have a capability of instantaneously relaxing the shear stress so as to prevent the cracking of the coating or the exfoliation of the coating from the printing substrate.
However, even when the foregoing ink satisfactory in stretching and adhesion is used, the ink is not yet of a sufficient level in such a way that in the punching processing of a printed matter, the cracking or the exfoliation of the ink coating occurs in the punched cross section; as affairs now stand, there are no ink compositions capable of satisfying the performances required in the market such as the stretching and bending processability and the punching processability of the substrate.
Accordingly, as described in Patent Literature 3, a photocurable inkjet printing ink composition has been proposed which indispensably includes a urethane oligomer having an ethylenically unsaturated double bond and isobornyl acrylate. The ink composition indispensably includes a urethane oligomer different from a monomer, and is thereby capable of improving the punching processability. However, an accumulated irradiation amount of light energy of 300 mj is required to achieve the curing, and the curability is sometimes insufficient.